When I "Add to Cart" from the product page, the product is added twice.

Adding to cart from the shop page adds 1 item as expected.

I searched on this and found a known conflict:
"Double add to cart issues.....
Check your page source for any empty background-image (css) properties, or img and link tags with empty src/href attributes. These can cause pages to load multiple times."

It happens with all products so I assume it must be with the template page. I have re-placed all of the cart and single product pages with no effect.

I have some template overrides working, but removing these does not fix the problem.